Is peptide histidine isoleucine an inhibitory nonadrenergic noncholinergic neurotransmitter in the rat gastric fundus?
In the rat gastric fundus, vasoactive intestinal polypeptide (VIP) contributes to the nonadrenergic noncholinergic (NANC) inhibitory neurotransmission and coexists with its related peptide, peptide histidine isoleucine (PHI). As the two neuropeptides are co-released and have similar actions in different biological systems, the effect of PHI in the rat gastric fundus was studied in order to investigate whether it might be a co-transmitter of VIP in the inhibitory NANC neurotransmission in this tissue. Auxotonic responses were measured in longitudinal muscle strips from the gastric fundus of reserpinized rats (5 mg/kg i.p., 24 hr before sacrifice), suspended between parallel platinum electrodes in Krebs solution containing atropine (1 microM) and 5-hydroxytryptamine (3 microM). PHI induced concentration-dependent relaxations (10 nM-1 microM), qualitatively similar to those induced by electrical field stimulation (1 msec, supramaximal voltage, 0.25-16 Hz) and by VIP (0.3-100 nM), although
